1. Field of the Invention
This invention relates to a method for disintegrating a hydrate polymer and a method for producing a water-absorbent resin while using a disintegrating step as part of the procedure thereof. More particularly, this invention relates to a method for disintegrating a hydrate polymer which, while disintegrating a hydrate polymer by using a screw extruder, executes the disintegration with water fed to the device, prevents the device from generating a kneading motion during the course of disintegration, and allows the product of the disintegrating to be arranged in a uniform thickness in a dryer, and a method for producing a water-absorbent resin while using a disintegrating step as part of the procedure thereof.
2. Description of the Related Art
The water-absorbent resin possesses the property of absorbing a large volume of water and has been used as materials for forming such sanitary articles as disposable diapers, sanitary napkins, and incontinence pants, a water retaining agent for soil, and drip absorbing sheets for foodstuffs. For use particularly in such sanitary articles as disposable diapers, the development of a water-absorbent resin having a high ratio of absorption capacity under high pressure has been yearned for allowing a decrease in the thickness of the product.
The water-absorbent resin of this kind is produced by polymerizing a monomer solution including acrylic acid or sodium acrylate etc. in the presence of a cross-linking agent and thereafter disintegrating the resultant polymer into a proper size.
Since the hydrate polymer contains water, the hydrate polymer is dried and pulverized in a target size after the drying process generally. Furthermore prior to the drying process, it is necessary for the hydrate polymer to be disintegrated so as to secure large surface of the polymer for the purpose of heightening the efficiency of drying. As means to disintegrate a hydrate gel-like polymer having a water content in the range of 50-80 wt. %, methods directed toward continuing a disintegration while supplying proper additives to the site of disintegrating such as, for example, a method which comprises supplying a detackifying agent having a boiling point in the range of 200-400° C. together with a gel to an extruding device (JP-B-1991-73576), a method which comprises supplying such an anti-adhesion agent as silicone oil to the contact surface of a cutting blade (JP-A-1998-87842), and a method which comprises supplying such a hydrophilic organic solvent as alcohol as a lubricant to the neighborhood of a rotary blade (JP-A-1999-35691) have been available. A method which comprises extruding a hydrate polymer in a thickness in the range of 1-20 mm through a perforated plate having a pore diameter in the range of 3-20 mm in due consideration of such factors as the friction between the wall surface of an extruding device and the hydrate gel-like polymer and disintegrating the resultant gel having a small water-soluble component and containing a small amount of a residual monomer (JP-A-1994-41319) and a method which comprises heating a hydrate polymer to a temperature in the range of 45-90° C. and extruding the hot hydrate polymer through a perforated plate having a pore diameter in the range of 6.5-18 mm (JP Patent No. 3145461) have been available. Further, as means to confer improvements on the devices, a method which comprises disintegrating a hydrate polymer while keeping this polymer nipped between opposed helical rotary blades having different feed rates (JP-A-1999-188727) and a method which comprises disintegrating a hydrate polymer by the use of a device provided near the extruding port thereof with a reverse-proofing member for the purpose of preventing the gel from flowing back (JP-A-2000-63527) have been available. They concern techniques for preventing the disintegrated hydrate polymer from adhering to the device or forming a conglomerate due to self-adhesion of the fact that a hydrate polymer having a water content in the range of 50-80 wt. % manifests stickiness and also concern techniques for preventing the hydrate gel-like polymer disintegrated with a meat chopper or a kneader from giving rise to a kneading motion. When the hydrate polymer during the course of disintegration is compressed by kneading, the resultant hydrate gel-like polymer is affected by such a powerful external mechanical force as to suffer breakage of the cross-linked polymer chain or increase the amount of a water-soluble component. The product of this disintegration possibly yields to self-adhesion and, during the course of drying, undergoes conglomeration and entails a decrease in the drying efficiency.
Meanwhile, the hydrate polymer having a water content in the range of 30-50wt. %, namely containing a solid component at a concentration in the range of 50-70 wt. %, is so difficult to handle because of its property that any attempt to disintegrate it to an extent of being directly dried in its unmodified form on a commercial scale has been made only in vain. Example 1 of JP-B-1991-73576 was barely capable of disintegrating a gel having a solid content of 33 wt. %, Example 1 of JP-A-1998-87842 a solid content of 25 wt. %, Example 1 of JP-A-1999-35691 a solid content of 30 wt. %, Example 1 of JP-A-1994-41319 a solid content 38 wt. %, Example 1 of JP Patent No. 3145461 a solid content of 38 wt. %, JP-A-1999-188727 succeeded in disintegrating a hydrate polymer having a water content of 39 wt. % but required use of an expensive device, and JP-A-2000-63527 used a gel having a solid component of 35 wt. %. Incidentally, when the solid content exceeds 82 wt. %, the hydrate polymer can be disintegrated by an ordinary shock type pulverizer similarly to a dried polymer.
When the monomer component which is a raw material compound for a water-absorbent resin is subjected to aqueous solution polymerization and the resultant polymer is deprived of the water contained therein, this removal of water necessitates time and thermal energy and rise the unit cost of production. Thus, attempts to develop a technique of producing a hydrate polymer of a high solid content and reducing this hydrate polymer to a finished commercial product have been continued. For example, a method for disintegrating a hydrate polymer having a high solid content in the range of 55-82 wt. % (water content 18-45 wt. %) as compared with the conventional hydrate polymer has been developed and a method for effecting expected disintegrating by the use of a vertical pulverizing device provided with a screen (JP-A-2002-212204). The method consist in increasing by not less than 2 points the solid content of the hydrate polymer having a solid content in the range of 55-82 wt. % by the use of the device mentioned above and/or disintegrating the hydrate polymer by passing a gas, preferably dry air, through the interior of the disintegrating device. The steam generated in the device tends to condense in the device and induce the hydrate polymer to adhere to and block the interior of the device but the ventilation serves to repress these adverse effects of the steam. None of the patent publications mentioned above report a case of disintegrating a hydrate polymer having a solid content in the range of 50-70 wt. % by the use of a screw extruder.
Among the group of disintegrating machines, a screw extruder is an inexpensive and compact machine. It may be readily adopted when it is capable of disintegrating a hydrate polymer having a solid content in the range of 50-70 wt. %. When this device is simply used for disintegrating the hydrate polymer, the product of disintegrating possibly undergo adhesion on the screw in motion. Once the adhesion between the product of disintegrating occurs, it will not be easily dispersed in a uniform thickness and will form a cause for degrading the efficiency of drying at the subsequent drying step. Further, when the particles resulting from the drying are mutually bound to form masses, these masses necessitate a pulverizing step and they possibly undergo pulverization to give rise to a fine powder. The occurrence of this fine powder results in degrading the quality of the product and increasing the loss of product.
When the hydrate polymer is disintegrated while it is kneaded and compressed during the course of disintegration, the hydrate polymer is exposed to a powerful mechanical force, with the result that it will cause breakage of the cross-linked polymer chain and increase the amount of the water-soluble component. Particularly when the hydrate polymer has been obtained by polymerizing a water-soluble ethylenically unsaturated monomer in the presence of a cross-linking agent so as to contain bubbles therein, the resultant water-absorbent resin possibly suffers the property thereof degraded because the bubbles contained therein are crushed by the kneading motion.
The water-absorbent resin is required to retain a prescribed absorption capacity and keep the contents of the residual monomer and water-soluble component below prescribed levels, depending on the kind of use and from the standpoint of the feeling of use. It is well known that the property of the water-absorbent resin is varied by the method for disintegrating. The development of a disintegration method which is capable of producing a hydrate polymer of high quality is yearned for.
This invention, therefore, is aimed at providing a method for disintegrating a hydrate polymer having a solid content in the range of 50-70 wt. %, which method, while executing the disintegrating by the use of a screw extruder, induces generation of a kneading motion and entails mutual adhesion of the product of disintegration only meagerly and enables this product to be dispersed in a uniform thickness in a dryer.